Microwave ovens are used in nearly 90% of U.S. households. The wide popularity of this appliance is well deserved since it delivers much of what is promised: faster and easier food preparation, cooler kitchens and easier clean-up.
An unexpected benefit of microwave cooking is that most types of burn injuries to children are much less frequent than from conventional ovens and stoves. There is, however, the disturbing finding of a class of injuries of increasing frequency with microwave oven use. These are burns to the oropharynx, palate and airway of infants fed from bottles that were heated in microwave ovens. Despite manufacturers' warning labels on baby bottle packages that discourage microwave heating and emphasize the potential risk, this problem persists.
There are several factors which contribute to this type of injury. Most significant are the uneven heating of the baby formula and the fact that the surface temperature is unrepresentative of the highest internal temperature. This situation would not result in burn injuries were it not for the care taker's failure to take the necessary steps to ensure safe delivery; namely, inverting the bottle several times to mix the contents to achieve uniform temperature, and testing the temperature of the liquid by dispensing a few drops on his/her skin.
Reflective electrically conducting materials opaque to microwave radiation have been used to affect cooking performance in microwave ovens in three ways. One application of these shields is to achieve differential heating when a plurality of foods is heated or cooked in a microwave oven. U.S. Pat. No. 3,865,301 describes a shielded container, opaque to microwave radiation except for radiation-transparent windows, used to heat a sandwich-type food comprised of a plurality of ingredients to different extents.
U.S. Pat. No. 4,851,631 to Wendt describes a food container with a cylindrical aluminum foil shield that protects the ice cream on a brownie from microwave radiation while the brownie is being heated.
U.S. Pat. No. 4,233,325 describes a two-component package containing a microwave reflective material which protects the ice cream in one compartment while a refrigerated syrup contained in a microwave transparent compartment is warmed.
Another application of metallic shields is to reduce the amount of microwave energy reaching a frozen food product by a controlled amount. Often the shield is combined with a glossy microwave absorber material so as to cook the food primarily by conduction heating. This is accomplished by shielding the major portion of the food product within the container from microwave radiation, while utilizing a layer of microwave absorber in contact with the food which heats the food as it absorbs radiation. This application of a shield with a microwave absorber is intended to enhance the organoleptic properties of the food.
U.S. Pat. No. 4,703,148 describes a package with sides made of an aluminum foil shield having windows whose size, number, and location are selected so as to achieve the desired level of crispness and browness.
U.S. Pat. No. 4,190,757 describes a metal foil laminated to Kraft paper bonded to the inside surface of a cardboard box for an individual size pizza pie. A predetermined number of openings are made in the shield so as to control the heating to result in a pizza with improved texture and appearance.
U.S. Pat. No. 3,941,967 describes a cooking apparatus containing a metallic shield capable of scorching a food. U.S. Pat. No. 4,351,997 to Mattisson et al. describes a food package containing side walls and rim coated with aluminum foil capable of cooking a composite frozen food product in a microwave oven to an even temperature with slight variations from 65.degree. C. to 80.degree. C.
U.S. Pat. No. 4,661,672 to Nakanaga describes an oblong container for use in microwave ovens comprising a shielding layer which covers the top of the contents and at least the upper half of the side walls on the short ends of the container which is capable of preventing hardening and drying of the corners of the contents, and allowing the contents to be uniformly and effectively heated.
Still another application of metallic shields is to achieve even heating.
U.S. Pat. No. 4,703,149 to Sugisawa et al. teaches that the top portion of food heated in a microwave oven is irradiated from both the top and sides causing the food to be heated unevenly. In a container according to this invention, a shielding layer is provided through the intermediary of an air layer at a position of the container where the shield covers at least the region where the upper surface of the contents make contact with the side surface of the container. The inventors found that interposing of an air layer between the microwave shielding layer and the container proper further increased heating efficiency and remarkably decreased induction heating in the shielding layer. It is alleged that with this design, sudden local boiling of the contents, such as soup, can be prevented.
U.S. Pat. No. 5,370,883 describes a package in a tray form for microwave heating of foods that provides an aluminum laminate for covering the side wall which allegedly gives excellent temperature distribution in microwave ovens. Despite the significant number of patents on the application of shields in the heating of food in microwave ovens, the use a foil reflector to microwave is clearly contraindicated in the technical literature. Shapiro and Bayne conclude their publication with oil labels covering a high percentage of the side wall area should not be used, since they prevent the penetration of microwave radiation through the jar and into the food, thereby inhibiting uniform heating."
For a considerable time manufacturers of microwave ovens have recommended that metallic shields not be introduced into microwave ovens because of potential damage to the magnetron and the potential for arcing that can damage the food package and char the food product. Arcing is a plasma arc discharge that produces a flash of light, a noise and sometimes ignition of the container.
The conductive shield can be a major source of arcing. Any discontinuity in the shield edge produces an intensification of the electric field emanating from that edge. At locations where the field strength is sufficiently large, an arc discharge will occur, and the heat produced in the arc may produce burning of adjacent portions of the food or the container. If the container is thermoplastic, it may deform or melt.
U.S. Pat. No. 3,865,301 to Pothier et al. describes design criteria for shielded containers to accomplish selective and controlled heating of foods in a microwave oven without arcing or charring. This patent teaches that it is desirable to reduce the number and sharpness of points in the conductive sheet. It also teaches that, in general, a conductive edge perpendicular to the shelf in a microwave oven is likely to produce arcing or charring. It further teaches that a single integral conductive sheet with no overlapped joints is more resistant to arcing then one piece with an overlapped joint.
U.S. Pat. No. 4,558,198 to Levendosky discloses a metal container and system for arc-free microwave cooking and minimal reflection of electromagnetic radiation. These benefits are achieved, according to the disclosure, by means of structural refinements in a metallic container, including the provision of smooth, wrinkle-free side and bottom walls and edges, a physical geometry incorporating generous radii in lieu of sharp corners in the container structure, and a coating of heat-resistant plastic material of a specified film thickness on both sides of the walls and edges of the container to diffuse microwave radiation.
U.S. Pat. No. 4,345,133 to Cherney et al. describes a partially-shielded microwave carton constructed such that adjacent portions of the panels forming the cover wall are provided with a low impedance electrical connection at microwave frequencies to inhibit arcing between such panels during heating and rounded to minimize the electric field intensity created at these corners-thus reducing the likelihood that arcing will occur between various portions of the cover, or between the cover and the surface on which the carton is supported.
U.S. Pat. No. 4,122,324 to Falk teaches that slight irregularities such as scratch mark or pinpoint in the shielding film can result in arcing and the attainment of temperatures in the region of the arc which far exceed the flash point of a combustible layer within the package. To avoid such problems, the patentee discloses that the sheet material from which the outer package is formed is coated or laminated on both surfaces with metal conductive layers so that no significant portion of the dielectric sheet is exposed to the oxygen in the air.
In order to practice the method if the instant invention it is desirable to measure the temperature within the container to be heated.
U.S. Pat. No. 4,156,365 to Heinmets et al. describes a thermochromic layer painted on the surface of a food vessel for indicating that the food content in the vessel has been heated above the minimum temperature of 60.degree. C. for ensuring the cessation of and the production of toxins of certain harmful microorganisms and below a maximum safe temperature of 70.degree. C. which can result in tissue damage to the lips, mouth and tongue if the food is ingested. No specific thermochromic composition for achieving this object is disclosed, nor does it indicate the method of heating, how the heating method might effect the thermochromic layer, or the usefulness of such an indicator in cases where the temperature is not uniform throughout the food.
U.S. Pat. No. 4,538,926 to Chretein describes a temperature indicating device for sensing the temperature of a bottle containing a liquid intended for human consumption. The temperature indicator of this invention is comprised of a cholesteric liquid crystal composition which undergoes a color change accompanying the broad temperature transition from the smectic to cholesteric phases printed on the bottle in the immediate vicinity of a heat insensitive color mark indicating the color corresponding to a temperature for optimal consumption of the liquid in the bottle.
U.S. Pat. No. 4,919,983 to Fremin describes an infant feeding bottle constructed of a thermoplastic containing a thermochromic microcapsular composition which undergoes a distinct change in color when the temperature is above the human range of comfort, about 36.degree. C. to about 38.degree. C. This Fremin '983 does not address the effect of microwave radiation on the thermochromic composition and on the additives for protection against UV radiation, nor does it address the problem of large temperature gradients in bottles containing liquids warmed in a microwave oven.
U.S. Pat. No. 4,878,588 to Ephraim describes a baby nursing bottle having a commercially available liquid crystal strip type thermometer disposed in its side wall. The size of this thermometer extends over the entire side of the baby bottle such that each discrete temperature sensor is located at a different height along the bottle. This invention is described to be reusable and stable after repeated microwave heating, cleaning in a dishwasher or sterilization with boiling water. It is the experience of the inventors of the present invention that commercial liquid crystal thermometers such as those described in Ephraim '588 are not stable under prolonged exposure to boiling water. The application of the type of thermometer disclosed in Ephraim '588 is of limited value if the bottle is heated in a microwave oven and the contents are not well mixed prior to the reading. A further limitation of liquid crystal strip thermometer made in accordance with the disclosures of Ephraim '588 is its failure to indicate whether the temperature at the surface to which it is applied is either above or below its range.
U.S. Pat. No. 3,864,976 to Parker describes a similar digital liquid crystal thermometer attached to a baby bottle with an elastomeric band having a thermometer structure of substantial flexibility so that it will conform to the container shape. Since the thermometer disclosed can be removed from the bottle during cleaning and sterilization, the commercial digital liquid crystal type thermometers are adequate for this application. Similar limitations, however, apply to its use discussed above. In conclusion, it is believed that the prior art on shielding has not addressed the means for enhancing mixing in liquids which are heated with microwave radiation such that substantially isothermal conditions are achieved throughout the liquid, therefore making useful a temperature indicator applied to the surface of the shielded bottle containing a liquid to be ingested by infants. Further, U.S. Pat. No. 4,851,631 which describes a metallic shield concentric with the container to be heated in a microwave oven teaches away from this invention by emphasizing that the geometry of a shield must be carefully selected to avoid resonance if the potential for arcing is to be eliminated. U.S. Pat. No. 4,703,149 also describes a cylindrically-concentric metallic shield, discloses that the microwave shield be separated from the container to be heated by an air gap.
U.S. Pat. No. 3,865,301 discloses that it is desirable to reduce the number and sharpness of points in the conductive sheet, that a conductive edge perpendicular to the shelf in a microwave oven is likely to produce arcing or charring and that a single integral conductive sheet with no overlapped joints is more resistant to arcing than one piece with an overlapped joint.
The prior art fails to address the problem of heating a baby bottle in a microwave oven so that its contents would reach a uniform temperature that would be faithfully represented by its surface temperature at any location, thereby securing the reliability of a decision regarding feeding safety based on external touch.